Liquid crystals have recently come into use in a variety of technical fields in which there is a requirement for certain electrooptical properties (for example low triggering voltages) combined with certain requirements concerning display or switching devices (for example flat construction, low weight). These devices currently utilize dielectric alignment effects in nematic, cholesteric and/or smectic liquid-crystal phases, the light transparency or reflectivity of the device being dependent on the electrical voltage applied.
A liquid-crystal display consists of two supporting plates, preferably glass plates, which are coated with transparent electrodes and, as a rule, with one or two alignment layers between which the liquid-crystal layer is located. Other components such as polarizers, color filters, passivating layers, anti-reflection layers, diffusion barrier layers and the like are in common use.
Although currently nematic or cholesteric liquid-crystal phases are still predominantly used, for some years ferroelectric, in particular smectic C*, liquid-crystal phases have been gaining in importance.
Ferroelectric liquid crystals have the advantage of very short response times and allow high-resolution screens to be operated without the assistance of electronic elements, such as for example thin-layer transistors, which are necessary when using nematic or cholesteric liquid-crystal phases.
In all the above applications, the liquid crystals are low-molecular-weight liquid-crystalline compounds, i.e. having molecular weights of below 2000 g/mol, preferably below 800 g/mol, and in particular they are not polymers, copolymers, polycondensates or copolycondensates. Owing to their low viscosity, low-molecular-weight liquid crystals generally have the advantage of short response times; this is particularly true of ferroelectric liquid crystals, whose response times are in the range of .mu.s and which therefore respond 10 to 1000 times faster than conventional nematic liquid-crystal phases.
However, on using ferroelectric liquid crystals, the problem of high susceptibility of the alignment to mechanical stress (shock, impact, pressure, neat distortion, bending and so on) can occur, which can lead to irreversible disruption of the image quality of a display. Currently, this high susceptibility impedes the construction of flexible ferroelectric LC displays and increases the cost of production of conventional displays, i.e. those with glass or rigid plastic plates.
It is advantageous to use polymeric liquid crystals owing to their lower deformability and better processibility.
Although polymeric liquid crystals have already been described on several occasions (for example J. Polym. Sci. Polym. Lett. Ed. 13,243 (1975); Polym. Bull. 6, 309 (1982)), the polymeric liquid crystals which have been described up till now have response times which are too long for practical purposes.
Ferroelectric, quick-response polymeric liquid crystals should therefore be particularly suitable for the production of flexible displays.
It would be particularly advantageous to produce a display film by a continuous process in which quick-response polymeric ferroelectric liquid crystals are incorporated. EP-0,228,703 employed a comprehensive general formula to describe a large number of ferroelectric, liquid-crystalline polymers. However, the response times of the examples mentioned were between 10 and 100 milliseconds.
The present invention accordingly provides ferroelectric, polymeric liquid crystals which fall within the general claim of the said application, but which contain in each side chain of the polymer as a particular structural feature a chiral carbon atom which is directly bonded via an oxygen atom to the phenyl ring of the mesogen. The polymers according to the invention have response times which are significantly shorter than those mentioned in EP-0,228,703.